Is forums.microsoft.com/technet no longer active? 
Because I cannot post this there, I am posting it here.

Windows 8.1 x64, AMD Phenom X3 2.3GHz, ASUS M4A785M,
Radeon 5450, 4GB Crucial PC2-6400, 111GB Mushkin Chronos SSD

File Sharing for MS Networks is always turned off.  I have temporarily enabled Sharing on the data volume (D:).  DEP is enabled only for essential Windows programs.
Protection is on for the system volume (C:) and off for the data volume (D:).

Avast! free anti-virus and Malwarebytes (free) show no infections or problems.  I am not aware of any previous infestation.

Without warning or explanation, why or what for, volume D: becomes read-only, except for the Recycle Bin.

From the top of the D: volume directory hierarchy to the bottom, the "Read-only" circle is filled, and it stays filled, whatever I do.  I uncheck "Read-only" for volume D: or any folder or file on D:, but without success. When I click "Apply" or "OK", the "Read-only" circle is filled again.
I grant "Full Control" to Users and Authenticated Users, just as SYSTEM and Administrators have.
I can modify files in "My Documents" on the data volume (D:), but not files in other folders.
I disable (and enable again) inheritances to subfolders and files, which remain read-only regardless.

Windows operates normally, even though the system volume (C:) is almost all read-only.
On C:, Bootmgr and all folders, including $RECYCLE.BIN, are read-only. 
"Read-only" is unchecked only for BOOTNXT, Log.txt, pagefile.sys, and swapfile.sys.

Being unable to change or modify files on my data volume (D:) has me at a standstill.

Thank you for any help you have to offer.

• Edited by gospelmidi 13 hours 46 minutes ago

Now when I uncheck the read-only attribute for each folder on D:, I can modify the files in that folder, even though the read-only attribute for the folder still appears to be on.  I have no idea what I changed that made modifying the files possible.  Volume D: is now completely unlocked for modifying and saving files. 

I didn't touch the Registry.  BTW, in File Explorer, the preview pane is off.

Might it be relevant that there are icons on the desktop for some of the folders on volume D:?

Because the Microsoft firewall is on, and File Sharing for MS Networks is off, I don't THINK there is a prankster intruder trespassing in my PC.  I suppose that is possible, but there's not much on my 111 GB SSD to interest a hacker - no SSN, no credit card numbers, no password list, no love letters...

The randomness of this on-again, off-again read-only locking of my own files and documents is maddening.  I never know when I begin editing a file whether I will be able to save it or not.  It's similar to writing a "dissertation" ;^) on a forum and clicking save, only to find out that the forum has timed out or logged me out.

But I suppose it's not random at all.  I just don't know what I changed.  When I find out, it will no longer seem random, but perfectly orderly.

• Edited by gospelmidi 3 hours 30 minutes ago

Locked out again.  Cannot save files.